In nuclear medicine imaging techniques such as single photon emission computed tomography (SPECT) and positron emission tomography (PET), medical images are regenerated based on radioactive signals, typically in the form of gamma rays, emitted from the body of a patient after the patient has been injected with a radiopharmaceutical substance. Emitted gamma rays are detected from numerous different projection angles by a gamma camera (i.e. Anger camera or scintillation camera) scanning around a longitudinal axis of the patient before conversion into electrical signals that are stored as data. Data from image projections provide a set of images as a result of a process known as image reconstruction.
Image reconstruction methods include iterative methods, such as Maximum-Likelihood-Expectation-Maximization (ML-EM) and Least Squares (LS), as well as traditional (non-iterative) reconstruction methods, such as filtered back-projection (FBP). Iterative reconstruction methods often provide better image quality and more natural ways to incorporate attenuation correction than non-iterative methods. However, iterative methods are generally more computationally intensive and more time-consuming than non-iterative methods. In fact, iterative techniques can be on the order of ten times slower than non-iterative techniques. Consequently, in the past iterative techniques were not used in clinical environment as sufficient computational power was cost prohibitive.
Over a period of approximately fifteen years a number of studies have addressed attenuation correction of images obtained using tomographic techniques. According to U.S. Pat. No. 5,376,795, photon attenuation constitutes a major deficiency in diagnosis of heart disease with SPECT and is a major source of error in the measurement of tumor metabolism using radionuclide techniques. A number of researchers have shown that use of emission-transmission imaging techniques overcomes these limitations by combining anatomical (structural) information from transmission images with physiological (functional) information from radionuclide emission images. By correlating the emission and transmission images, the observer can more easily identify and delineate the location of radionuclide uptake. In addition, improvement of the quantitative accuracy of measurement of radionuclide uptake is possible using iterative reconstruction methods, which can account for errors and improve the radionuclide images.
Other studies in this area include disclosures of U.S. Pat. No. 5,739,539, which describes a method of performing image reconstruction in a gamma camera system that includes the steps of performing a transmission scan of an object about a number of rotation angles to collect transmission projection data and performing an emission scan of the object about numerous rotation angles to collect emission projection data. The outer boundary of the object is then located based on the transmission projection data. Information identifying the boundary is then either stored in a separate body contour map or embedded in an attenuation map. Information identifying the boundary can be in the form of flags indicating whether individual pixels are inside or outside the boundary of the object. The emission projection data is then reconstructed using the attenuation map, if desired, to generate transverse slice images. Image reconstruction requires less time if the process ignores pixels outside the body boundary.
U.S. Pat. No. 6,856,666 describes multi modality imaging methods and apparatus for scanning an object in a first modality, having a first field of view to obtain first modality data including fully sampled field of view data and partially sampled field of view data. The method also includes scanning the object in a second modality having a second field of view larger than the first field of view to obtain second modality data, and reconstructing an image of the object using the second modality data and the first modality partially sampled field of view data.
The below discussed additional U.S. patents are cited as being exemplary of the prior art in the technological field of the present invention.
U.S. Pat. No. 6,830,580 entitled PET and SPECT systems with attenuation correction describes a nuclear imaging system comprising: first and second radiation detectors, each comprising an imaging surface facing each other and each having an extent; an radiation source, situated outside a space defined by a largest parallelepiped formed on two sides by said first and second detectors, which irradiates the second detector; and an axis about which the first and second detectors and the radiation source rotate together; wherein the field of view of the radiation source, defined by lines connecting the external source and the edges of the second detector, encompass the axis of rotation.
U.S. Pat. No. 6,740,883: Application of scatter and attenuation correction to emission tomography images using inferred anatomy from atlas describes a method of applying scatter and attenuation correction to emission tomography images of a region of interest of a patient under observation comprises the steps of aligning a three-dimensional computer model representing the density distribution within the region of interest with the emission tomography images. The computer model is created from image data of other subjects thereby to avoid the need to image the subject under observation to create the computer model. Scatter and attenuation correction is applied to the emission tomography images using the aligned computer model as a guide.
U.S. Pat. No. 6,642,523: PET and SPECT systems with attenuation correction describes a nuclear imaging system comprising: first and second radiation detectors, each comprising an imaging surface facing each other and each having an extent; an radiation source, situated outside a space defined by a largest parallelepiped formed on two sides by said first and second detectors, which irradiates the second detector; and an axis about which the first and second detectors and the radiation source rotate together; wherein the field of view of the radiation source, defined by lines connecting the external source and the edges of the second detector, encompass the axis of rotation.
U.S. Pat. No. 6,631,284: Combined PET and X-ray CT tomography describes a combined PET and X-Ray CT tomograph for acquiring CT and PET images sequentially in a single device, overcoming alignment problems due to internal organ movement, variations in scanner bed profile, and positioning of the patient for the scan. In order to achieve good signal-to-noise (SNR) for imaging any region of the body, an improvement to both the CT-based attenuation correction procedure and the uniformity of the noise structure in the PET emission scan is provided. The PET/CT scanner includes an X-ray CT and two arrays of PET detectors mounted on a single support within the same gantry, and rotate the support to acquire a full projection data set for both imaging modalities. The tomograph acquires functional and anatomical images which are accurately co-registered, without the use of external markers or internal landmarks.
U.S. Pat. No. 6,490,476: Combined PET and X-ray CT tomograph and method for using same describes a combined PET and X-Ray CT tomograph for acquiring CT and PET images sequentially in a single device, overcoming alignment problems due to internal organ movement, variations in scanner bed profile, and positioning of the patient for the scan. In order to achieve good signal-to-noise (SNR) for imaging any region of the body, an improvement to both the CT-based attenuation correction procedure and the uniformity of the noise structure in the PET emission scan is provided. The PET/CT scanner includes an X-ray CT and two arrays of PET detectors mounted on a single support within the same gantry, and rotate the support to acquire a full projection data set for both imaging modalities. The tomograph acquires functional and anatomical images which are accurately co-registered, without the use of external markers or internal landmarks.
U.S. Pat. No. 6,339,652: Source-assisted attenuation correction for emission computed tomography describes a method of ML-EM image reconstruction is provided for use in connection with a diagnostic imaging apparatus (10) that generates projection data. The method includes collecting projection data, including measured emission projection data and measured transmission projection data. Optionally, the measured transmission projection data is truncated. An initial emission map and attenuation map are assumed. The emission map and the attenuation map are iteratively updated. With each iteration, the emission map is recalculated by taking a previous emission map and adjusting it based upon: (i) the measured emission projection data; (ii) a reprojection of the previous emission map which is carried out with a multi-dimensional projection model; and, (iii) a reprojection of the attenuation map. As well, with each iteration, the attenuation map is recalculated by taking a previous attenuation map and adjusting it based upon: (i) the measured emission projection data; (ii) a reprojection of the previous emission map which is carried out with the multi-dimensional projection model; and (iii) measured transmission projection data.
U.S. Pat. No. 6,249,003: Imaging attenuation correction method employing multiple energy scan masks and windows describes an apparatus for generating gamma transmission and gamma emission images simultaneously includes a camera and a line transmission source of gamma radiation disposed on opposite sides of an imaging area in which a patient lies. The line detector moves along a path that substantially traverses the field of view of the gamma camera. As the gamma camera moves an acceptance region and a mask region are electronically defined and moved across the camera's field of view. Photons striking the camera are categorized according to their energy and the region of impingement. The categorization defined an emission image, a transmission image and a crosstalk image. The transmission and crosstalk images are combined to form a corrected transmission image that then is used along with the emission image to produce and image of the patient.
U.S. Pat. No. 6,140,649: Imaging attenuation correction employing simultaneous transmission/emission scanning describes a nuclear medical imaging system generates transmission and emission images simultaneously. The system includes a gamma camera and a linear transmission source disposed on opposite sides of an imaging region in which a patient lies. A plurality of views are taken at different rotational angles around a patient. At each angle, the view acquisition period is divided into two segments based on whether the transmission source is on or off. Emission image data is acquired either in both period segments or only while the transmission source is off. The transmission image data is acquired when the transmission source is on, and crosstalk image data is acquired when the transmission source is off.
U.S. Pat. No. 5,959,300: Attenuation correction in a medical imaging system using computed path lengths and attenuation values of a model attenuation medium describes a method of correcting for attenuation during emission imaging in a gamma camera medical imaging system. Attenuation values are determined empirically and are stored in a look-up table in a memory that is readable by the imaging system, with each attenuation value corresponding to a given thickness value. The attenuation values are computed before imaging is performed by first measuring the number of photons which pass from a transmission source through various known depths of water or another suitable model attenuator, using the same radiation source as will be used for emission imaging. For each depth, the measurement is then used to compute the actual attenuation for a thickness of the model attenuator. The attenuation is then stored as a value in the look-up table with corresponding values of attenuator thickness and is later used to correct emission data for the effects of attenuation.
U.S. Pat. No. 5,338,936: Simultaneous transmission and emission converging tomography describes a SPECT system includes three gamma camera heads (22a), (22b), (22c) which are mounted to a gantry (20) for rotation about a subject (12). The subject is injected with a source of emission radiation, which emission radiation is received by the camera heads. Transmission radiation from a transmission radiation source (30) is truncated to pass through a central portion of the subject but not peripheral portions and is received by one of the camera heads (22a) concurrently with the emission data. As the heads and radiation source rotate, the transmitted radiation passes through different parts or none of the peripheral portions at different angular orientations. An ultrasonic range arranger (152) measures an actual periphery of the subject. Attenuation properties of the subject are determined by reconstructing (90″) the transmission data using an iterative approximation technique and the measured actual subject periphery. The actual periphery is used in the reconstruction process to reduce artifacts attributable to radiation truncation and the associated incomplete sampling of the peripheral portions. An emission reconstruction processor (112) reconstructs the emission projection data and attenuation properties into an attenuation corrected distribution of emission radiation sources in the subject.
U.S. Pat. No. 6,950,494: Method for converting CT data to linear attenuation coefficient map data describes a method for converting output data from a computer tomography (CT) device to linear attenuation coefficient data includes a step of receiving output pixel data from a CT device for a pixel of a CT image. The value of the pixel data is compared to a predetermined range. If the value is within the predetermined range, a linear attenuation coefficient is calculated from the pixel data using a first conversion function corresponding to said predetermined range. If the value is outside the predetermined range, the linear attenuation coefficient is calculated from the pixel data using a second conversion function corresponding to a range outside said predetermined range. The calculated coefficient is stored in a memory as part of a linear attenuation coefficient map.
Each improvement in attenuation correction of nuclear medicine images provides benefits associated with the quality of medical diagnoses. For this reason there is continuing need for methods of image reconstruction for reliable reproduction of a patient's physical and functional condition.